A. Field of the Invention
The present invention relates generally a cardiac stimulating device that combines the abilities and objectives of implantable pacemakers and defibrillators. More particularly, the present invention relates to a implantable cardioverter/defibrillator that is capable of both rate responsive pacing and detection of fibrillation. Still more particularly, the present invention relates to a cardiac stimulating device that includes a rate-responsive adjustment of the T-wave window, which is in turn used to automatically adjust the sensitivity, or gain of the device.
B. Description of the Related Art
In the normal human heart, illustrated in FIG. 1, the sinus (or sinoatrial (SA)) node generally located near the junction of the superior vena cava and the right atrium constitutes the primary natural pacemaker by which rhythmic electrical excitation is developed. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers (or atria) at the right and left sides of the heart. In response to excitation from the SA node, the atria contract, pumping blood from those chambers into the respective ventricular chambers (or ventricles). The impulse is transmitted to the ventricles through the atrioventricular (AV) node, and via a conduction system comprising the bundle of His, or common bundle, the right and left bundle branches, and the Purkinje fibers. The transmitted impulse causes the ventricles to contract, the right ventricle pumping unoxygenated blood through the pulmonary artery to the lungs, and the left ventricle pumping oxygenated (arterial) blood through the aorta and the lesser arteries to the body. The right atrium receives the unoxygenated (venous) blood. The blood oxygenated by the lungs is carried via the pulmonary veins to the left atrium.
This action is repeated in a rhythmic cardiac cycle in which the atrial and ventricular chambers alternately contract and pump, then relax and fill. Four one-way valves, between the atrial and ventricular chambers in the right and left sides of the heart (the tricuspid valve and the mitral valve, respectively), and at the exits of the right and left ventricles (the pulmonic and aortic valves, respectively, not shown) prevent backflow of the blood as it moves through the heart and the circulatory system.
The sinus node is spontaneously rhythmic, and the cardiac rhythm it generates is termed normal sinus rhythm ("NSR") or simply sinus rhythm. This capacity to produce spontaneous cardiac impulses is called rhythmicity, or automaticity. Some other cardiac tissues possess rhythmicity and hence constitute secondary natural pacemakers, but the sinus node is the primary natural pacemaker because it spontaneously generates electrical pulses at a faster rate. The secondary pacemakers tend to be inhibited by the more rapid rate at which impulses are generated by the sinus node.
Disruption of the natural pacing and propagation system as a result of aging or disease is commonly treated by artificial cardiac pacing, by which rhythmic electrical discharges are applied to the heart at a desired rate from an artificial pacemaker. An artificial pacemaker (or "pacer" as it is commonly labeled) is a medical device that delivers electrical pulses to an electrode that stimulates the heart so that it will contract and beat at a desired rate. If the body's natural pacemaker performs correctly, blood is oxygenated in the lungs and efficiently pumped by the heart to the body's oxygen-demanding tissues. However, when the body's natural pacemaker malfunctions, an implantable pacemaker often is required to properly stimulate the heart. Implanted pacemakers can either pace continuously, or can be provided with sensors that are able to detect a natural pacing signal, allowing the implanted device to be inhibited from pacing when artificial pacing stimulus is not needed. Pacing in this manner is known as demand pacing, in that the pacer only provides a pacing stimulus when it fails to detect a natural pacing signal.
In addition, pacers today are often rate responsive on the basis of sensor input. That is, one or more physiological parameters are measured and used as a basis for calculating an optimal pacing rate. Thus, the rate at which this type of rate responsive demand pacer looks for natural pacing signals depends on the value of the physiological parameter(s) being tracked. This produces an effect similar to that in dual chamber pacemakers in which the ventricular pacing rate increases as it "tracks" increases in the atrial rate. An in-depth explanation of certain cardiac physiology and pacemaker theory of operation is provided in U.S. Pat. No. 4,830,006. Throughout the following discussion and claims, the concepts of "rate responsiveness" and "pacing demand" refer to and include both sensor-driven and "tracked" types of systems.
Cardiac ventricular fibrillation is a condition characterized by rapid, chaotic electrical and mechanical activity of the heart's excitable myocardial tissue that results in uncoordinated activity of the heart tissue and consequent ineffectual quivering of the ventricles. This in turn causes an instantaneous cessation of blood flow from the heart. Unless cardiac output is restored almost immediately after the onset of ventricular fibrillation, tissue begins to die for lack of oxygenated blood and patient death can occur within minutes.
Defibrillation is a technique involving the application of one or more high energy electrical stimuli to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections and to restore the synchronized contraction of the total mass of heart tissue. Successful defibrillation requires the delivery of a sufficient electrical pulse to the heart of the patient to terminate fibrillation and preclude immediate re-emergence of the condition.
The use of implantable defibrillators for treating cardiac fibrillation is well known. Likewise, devices capable of delivering both demand pacing pulses and defibrillation shocks are known and are indicated for heart patients suffering from both irregular heart beat and occasional cardiac fibrillation.
Conventional pacemakers and implantable cardioverter/defibrillators (ICD's) typically operate by analyzing the electrical output of the heart in a series of windows. Referring now to FIG. 2, a portion of an external electrocardiograph showing two complete pacing cycles is labeled so as to illustrate the component parts of each cycle. A cycle begins with a P-wave, which indicates an atrial event, followed by a QRS wave indicative of ventricular contraction. The cycle ends with a T-wave caused by repolarization of the ventricles in preparation for the next cycle. A ventricular demand pacemaker looks for the R-wave that indicates a natural pacing or "sensed" event. When a sensed event occurs, the pacer is inhibited from providing an unnecessary pacing stimulus.
It is common for pacers and ICD's to use a ventricular pace refractory period (VPRP), during which ventricular activity is ignored. The VPRP helps prevent misinterpretation of post-pace electrical activity or T-waves as R-waves and creates a monitoring window for T-wave activity. Early pacers commonly used a fixed VPRP. Many dual-chamber and rate-responsive pacers still use a fixed VPRP, however, the VPRP in these devices is limited by the maximum possible ventricular pacing rate. That is, the VPRP must be shorter than the shortest possible pacing interval. Thus, the fixed VPRP may not be completely appropriate for all of the pacing rates that the pacer can deliver. To address this problem, it is known to dynamically adjust the VPRP in proportion to the pacing interval, allowing a longer VPRP to be used at slower pacing rates.
On the other hand, because the amplitude of the signals generated by the ventricles during fibrillation are so much smaller than the amplitude of signals generated during normal operation (either paced or sensed), there is a possibility that device may not be able to detect fibrillation signals during rapid ventricular pacing. If the device cannot detect fibrillation signals and thereby distinguish fibrillation from bradycardia, there is a possibility that the needed defibrillation therapy will not be administered. One solution to this problem is to adjust the gain of the sensing amplifier in response to changes in the amplitude of T-wave. Thus, it is desirable to use T-wave amplitude and/or timing data collected during the T-wave monitoring window within the VPRP to calculate, among other things, the degree to which gain should be adjusted. Because the physiologic delay from pace to T-wave is often proportional to the pacing interval, at slower ventricular pacing rates, the T-wave may occur too late in the fixed VPRP to be completely detected if the fixed VPRP is too short.
Thus, many of the approaches taken to address the problems related to rate-responsive pacing also have the undesired effect of reducing the reliability of the T-wave data. Hence, it is desired to provide a technique for ensuring that collection of T-wave data is optimized for all possible pacing/defibrillating modes and at all foreseeable pacing rates.